The balancing tank and the overflow swimming pool system always work together. The first one is located on the hydraulic circuit of an in-ground or semi in-ground pool, between the pool and the pump. Some people call it a surge or balance tank or a catch basin.

What Does A Balancing Tank In An Overflow Pool

In a traditional pool, the water goes through the skimmer; after filtration and possibly treatment and heating, it returns to the pool through the inlets.
The water circulation is slightly different, whether in an infinity pool or a zero deck level pool. A bottom drain equips the balance tank; it allows the pump to suck up the water and send it to the pool through the filtration circuit. In a zero-edge pool, the water’s top is at the same level as the edges surrounding the pool; water spills over the pool perimeter and goes to a channel. Water flows over one edge in a negative edge pool, like a waterfall.
No matter how the pool overflows, the balance tank is a key part of the overflow system.
The surge tank is designed to maintain a constant water level in the pool. Normal evaporation, number of swimmers, rain modify the water level in the swimming pool; The balance tank balances fluctuations and peaks.
Therefore and thanks to the balancing tank, the hydraulic circuit of the pool is also closed.

The Different Kinds Of Balancing Tanks

There are mainly two kinds of balancing tanks for overflowing swimming pools. The choice depends on whether the pool is built by a professional or not.

Built-in balancing tank

For a large overflow pool, the balancing tank, also named surge tank, is generally built in concrete in the pump room. For a smaller pool, the overflow gutter can be used as a balancing tank. These 2 solutions require a watertight seal to be applied to the bottom and walls of the tank.  Using a concrete surge tank for a large pool offers several advantages. Firstly, concrete provides a durable and long-lasting structure that can withstand the constant exposure to water and chemicals. Additionally, a concrete balancing tank allows for precise customization and can be designed to accommodate specific pool dimensions and water flow requirements. Finally, the watertight seal applied to the bottom and walls ensures that the buffer tank effectively prevents water leakage, providing a reliable solution for pool designers and builders.

Prefabricated tank

Many suppliers of tanks to be buried (and not especially pool equipment manufacturers) can provide such cisterns. There can be prefabricated concrete or molded high-density polyethylene tanks.
Suppose the site configuration does not allow the burying of the balance tank. In that case, it is possible to find even more economical solutions in plastic.


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Making the surge tank the right size

Suppose the balancing tank of a swimming pool is too small, the water will be washed away (and therefore the chemicals with it!); this can happen when the pool will be used by more swimmers or when it rains, for example. By oversizing, it, you will spend money for nothing. 

Don’t forget the security features

Not having safety features in a pool surge tank can lead to various issues. For example, it may not be able to handle sudden surges in water volume.

How To Calculate The Balancing Tank Capacity?

You have probably already read that the volume of the balancing tank should be between 5 and 10% of the volume of the pool. This is the theory, but several factors come into play.
  • The volume of the pool,
  • the overflow kind: You can choose a perimeter with the surface water flowing over the perimeter or one edge (generally a length) lower than the deck level to give an infinity illusion,
  • the overflow length,
  • the expected number of swimmers; this is a question of whether your pool is for strictly private use or whether you expect to have numerous people in it,
  • the flow rate of the pump and the filter diameter,
  • the presence of an auxiliary pump, which improves the overflow effect by increasing the thickness of the water layer.

The key components required for the operation of the balance tank

The balancing tank alone cannot do its job. The design of the filtration system must also include :

  • An overflow to prevent the tank from overflowing in the plant room when there are many swimmers in the pool or when it rains. It is usually connected to the main sewer.
  • A check valve to prevent the pool from draining by this balancing tank overflow when the pump is stopped. However, if the pump stops when this non-return valve is slightly open, the pool water will return to the balancing tank by gravity and when the water level reaches the tank overflow pipe, the water will flow into the drain. In this way, a pool can be siphoned off more or less completely, depending on the level of the overflow.
  •  A water level control system is crucial; it acts in both cases: lack of water or excess of water in the balancing tank.
  • The regulation system controls a water inlet with a solenoid valve. The water is topped up automatically according to water losses due to evaporation, splashes from swimmers, etc…
  • A suction hose between the balancing tank and the pump; that’s obvious, but it doesn’t cost anything to say it.

What Is The Price Of A Balancing Tank

For a prefabricated high density polyethylene balancing tank of 1 gal (3.79 l) US, you need to plan around $1,500. The same prefabricated concrete tank will cost you only $1,000. A concrete tank built in the pump room simultaneously as the swimming pool will have to be included in the total estimate; the price range is between $2,500 and $3,500 for the same volume. It includes the cost of the waterproofing of the bottom and walls of the tank. The gutter as a balance tank is a slightly cheaper solution if you agree to leave it open.
The cheapest option is an unburied catch basin.
Beware of cheap plastic tanks that can only be buried with various precautions: a blocks wall to be built around the tank or a stabilized backfilling (sand/cement). At the exit, this option could be more expensive.


The balancing tank for the overflow swimming pool must always be accessible; it must therefore have an opening (maintenance hole type) for possible cleaning. A basket or a protective net judiciously placed at the channel’s exit will block the biggest dirt, toys, … and will avoid you from plunging into it. It is easier to clean a simple gutter than to go down into the balancing tank of an overflow pool to clean it.
Some swimming pool owners install the check valve in the balance tank; we do not recommend it at all. It makes cleaning the check valve much more difficult.
When installing the catch basin, make sure that the overflow, the water suction hose to the pump, and the hose for the balancing tank leveler devices are correctly installed.